yuvPlanartoyuy2_c: fix sign extension

Fixes CID732282
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libswscale/rgb2rgb_template.c

@@ -353,9 +353,9 @@ static inline void yuvPlanartoyuy2_c(const uint8_t *ysrc, const uint8_t *usrc,
         const uint8_t *yc = ysrc, *uc = usrc, *vc = vsrc;
         for (i = 0; i < chromWidth; i += 2) {
             uint64_t k = yc[0] + (uc[0] << 8) +
-                         (yc[1] << 16) + (vc[0] << 24);
+                         (yc[1] << 16) + (unsigned)(vc[0] << 24);
             uint64_t l = yc[2] + (uc[1] << 8) +
-                         (yc[3] << 16) + (vc[1] << 24);
+                         (yc[3] << 16) + (unsigned)(vc[1] << 24);
             *ldst++ = k + (l << 32);
             yc     += 4;
             uc     += 2;